Organic light-emitting diodes (OLEDs), based on self-luminescence, enjoy the advantage of having a wide viewing angle and being able to be made thinner and lighter than liquid crystal displays (LCDs). In addition, an OLED display exhibits a very fast response time. Accordingly, OLEDs find applications in the illumination field as well as the full-color display field.
Materials used as organic layers in OLEDs may be divided into luminescent materials and charge transport materials, for example, a hole injection material, a hole transport material, an electron injection material, and an electron transport material. As for the luminescent materials, there are two main families of OLED: those based on small molecules and those employing polymers. The light-emitting mechanism forms the basis for classification of the luminescent materials as fluorescent or phosphorescent materials, which use excitons in singlet and triplet states, respectively. Further, luminescent materials may be divided according to color into blue, green, and red light-emitting materials. Furthermore, yellow and reddish yellow light-emitting materials have been developed in order to achieve more natural colors.
Meanwhile, when a single material is employed as the luminescent material, intermolecular actions cause the wavelength of maximum luminescence to shift toward a longer wavelength, resulting in reduced color purity and light emission efficiency. In this regard, a host-dopant system may be used as a luminescent material so as to increase the color purity and the light emission efficiency through energy transfer.
This is based on the principle whereby, when a dopant is smaller in energy band gap than a host accounting for the light-emitting layer, the addition of a small amount of the dopant to the host generates excitons from the light-emitting layer so that the excitons are transported to the dopant, emitting light at high efficiency. Here, light of desired wavelengths can be obtained depending on the kind of dopant because the wavelength of the host moves to the wavelength range of the dopant.
With regard to related arts pertaining to dopant compounds in the light-emitting layer, reference may be made to Korean Patent Publication No. 10-2008-0015865 (Feb. 20, 2008), which describes an organic light-emitting diode using an arylamine-coupled indenofluorene derivative, and Korean Patent Publication No. 10-2012-0047706 (May 14, 2012), which describes an organic photoelectric device using a compound in which dibenzofuran or dibenzothiophene coexists with fluorene or carbazole.
Further, and Korean Patent Application No. 10-2013-0121947 (Oct. 14, 2013), pertaining to a related art of host compound in the light-emitting layer, discloses an organic light-emitting diode employing an anthracene derivative as a fluorescent host.
Despite enormous efforts including the documents describing the related art, there is still the continued need to develop organic light-emitting diodes that exhibit lower driving voltages and longer lifespan.